It is frequently necessary to provide an explosive charge with a fuse that sets it off automatically when the charge is at a predetermined level. Although such a fuse can operate in accordance with barometric pressure to set off a charge when a certain altitude has been reached, it is more common for this type of fuse to find application underwater. Thus the fuse is provided on a depth charge usable in military operations, or on a charge used for generating shocks to make seismographs for geological or underwater-prospecting tasks.
The key requirements for such a fuse are that it operate surely to detonate the charge except when this level is reached. Thus elaborate precautions are take to prevent arming of the charge or operation of the fuse before use, and to prevent the charge from going off except under the exact desired circumstances. Furthermore it is essential that the fuse not detonate the charge when for some reason circumstances are not what they should be, as when the explosive is dropped in water which is insufficiently deep to activate the fuse but which later becomes deeper, as when the tide comes in, or when some part of the fuse mechanism fails.
A classic prior-art fuse can be seen in U.S. Pat. No. 4,056,058 issued Nov. 1, 1977 to Fernando Almarza Laguna de Rins. This device has a firing member which can be released to strike against a detonator by mechanism controlled by a diaphragm. The diaphragm is urged outwardly by a spring which holds it in a blocking position inhibiting movement of the firing member, but can be pressed inwardly by fluid pressure from outside. Thus if the fuse is dropped in the water the increasing pressure on the outside of the diaphragm will eventually overcome the spring force and free the firing member to detonate the primer.
Such an arrangement has the advantage of simple mechanism, and can easily be adjusted to go off at virtually any desired depth. Some disadvantages do remain, however. First of all, if the fuse is jarred or dropped so that a strong component of force is effective on the membrane and the associated structure in a direction displacing it inwardly, the fuse will operate. Secondly, the diaphragm is relatively exposed so that it can be damaged easily, making the device inoperative. Finally, virtually any fault in assembly or construction can make the device go off accidentally.